There are numerous applications in the field of fluid bed reactors operating under pressure where it is desired to withdraw solids continuously to a lower pressure. Heretofore, control of such withdrawal has been by way of valved lock hoppers, riser pipes, etc. For example, see U.S. Pat. No. 4,106,210 which discloses discharge of solids through an upwardly directed riser pipe described as being of a tapered design to maintain uniform rates of flow.
One of the features of the present invention is the use of a downwardly directed conduit or standpipe of non-uniform cross-section. Per se, such a conduit is known from U.S. Pat. No. 2,943,994 for quenching the temperature of a gas and from U.S. Pat. No. 3,661,799 wherein the conduit is a petroleum cracking reactor.